This invention relates to a holographic recording medium and to the composition of this medium which provides for a condition of low volume shrinkage, good recording sensitivity, and high dynamic range.
In prior art processes for formation of volume-phase holograms, data is stored as holograms resulting from the interference of signal and reference beams within a holographic recording medium comprising a homogenous mixture of at least one polymerizable monomer or oligomer and a polymeric binder; the polymerizable monomer or oligomer must of course be sensitive or sensitized to the radiation used to form the interference fringes. In the illuminated regions of the interference pattern, the monomer or oligomer undergoes polymerization to form a polymer that has a refractive index different from that of the binder. Diffusion of the monomer or oligomer into the illuminated regions, with consequent chemical segregation of binder from these areas and alteration in its concentration in the non-illuminated regions, produces spatial separation between the polymer formed from the monomer or oligomer and the binder, thereby providing the refractive index modulation needed to form a hologram. Typically, after the holographic exposure, a post-imaging blanket exposure of the medium to actinic radiation is required to complete the polymerization of the monomer or oligomer and fix the hologram. When holograms are multiplexed co-locationally, such as by multiple holographic exposures at different angle conditions, a post-imaging blanket exposure of the medium to actinic radiation may also be required to complete the polymerization of the monomer or oligomer and fix the multiplexed holograms.
One important potential use of volume holograms is in digital data storage; the three dimensional nature of a volume hologram, which refers to the storage of each bit as a hologram extending throughout the entire volume of the recording medium, renders volume holograms suitable for use in high capacity digital data storage. A group of bits can be encoded and decoded together as a two dimensional array of bits referred to as a page. Various multiplexing methods, such as angular, spatioangular, shift, wavelength, phase-code, and related methods, are used to store multiple pages co-locationally within the same volume or in partially overlapping volumes.
Photopolymerizable holographic recording media for write-once-read-many (WORM) data storage applications should ideally exhibit pre-recording shelf life of at least a year, good recording sensitivity, high degree of optical homogeneity (i.e. low scattering), uniform recording characteristics, stable image fidelity, and low volume shrinkage coupled with high dynamic range or cumulative grating strength. Low volume shrinkage coupled with good dynamic range and recording sensitivity, however, remains as one of the most difficult to achieve performance characteristics for photopolymerizable holographic recording media that are designed for data storage applications. Typically, high dynamic range is achievable with photopolymer recording materials but the resultant volume shrinkage is significant and thus poor image fidelity and poor signal to noise results. Uh-Sock Rhee et al. in Applied Optics, 34, 5, 846 (1995) describe the shrinkage effect in Dupont HRF-150-38 photopolymer as a function of increasing exposure. This material comprises monomers that are photopolymerized using conventional free radical polymerization chemistry. The magnitude of the deviation of the Bragg angle, displayed in FIGS. 6 and 7 of the above reference, is about 2.5xc2x0. This value is five times larger than the full width at half maximum (FWHM) of the Bragg peak for a plane-wave hologram with a grating period in the intermediate range of a digital page based image, and which is recorded in a medium thickness of only 100 xcexcm. Accordingly, even for such a thin recording medium, which is not sufficiently thick for a holographic data storage medium, the angle shift from the recording condition exhibited by the Dupont HRF-150-38 material is so large that no diffraction efficiency is observed at the recording angle condition, and thus an image could not be reconstructed without substantially tuning the angle of the read beam. Tuning the angle of the read beam is not desirable for a holographic data storage system since this imposes significant overhead on the readout design and would seriously impair data rates. Furthermore, a single tuning adjustment would be inadequate to read an image affected by such levels of shrinkage, since page based images comprise a continuum of plane-wave grating components with a range of grating angles. Contributions to an image from larger grating angle components would be shifted further in angle from the Bragg condition than for smaller grating angle components, and thus differential tuning would be required to reconstruct the high and low frequency features of an image. For thicker recording media this problem is exacerbated further due to the inverse proportionality of FWHM on media thickness.
A consequence of high volume shrinkage is the requirement to use up a portion of the reactive monomeric or oligomeric species in the recording medium before holographic recording of information can be implemented with reasonable fidelity. This step has the effect of diminishing the usable dynamic range as well as causing significant decline in recording sensitivity. An example of this tradeoff is the photopolymer material developed by Lucent Technologies, and which is based upon conventional free radical polymerization chemistry. By way of example, this material comprises difunctional acrylate oligomers and monomers such as N-vinyl carbazole and isobornyl acrylate (see Dhar et al. in Optics Letters, 23, 21, 1710 (1998), all of which exhibit significant volume shrinkage upon polymerization. Accordingly, Dhar et al. describe that if more than twenty 480-kbit images are recorded co-locationally in this material, for either 250 or 500 xcexcm thick media, then the raw bit-error rate (BER) rises above the desired upper limit value of 5e-3. This occurs due to excessive volume shrinkage, which firstly reduces image fidelity and secondly causes an increased degree of Bragg detuning as the extent of polymerization increases. In Optics Letters, 24, 7, 487 (1999), Dhar et al. describe a photopolymer recording media that exhibits moderate dynamic range per unit thickness but which suffers from significant volume shrinkage. Accordingly, Dhar et al. show that the cumulative grating strength for a material that exhibits 0.5% transverse shrinkage declines by a factor of about 4 to 5 when the concentration of monomer is reduced in order to achieve an improved and minimally desirable value of 0.2% transverse shrinkage. Consequently, for 200 xcexcm thick media Dhar describes that the cumulative grating strength diminishes from about 9 to about 2, an unacceptably low value, when a preconditioning step is used to consume a portion of the monomeric species in order to reduce shrinkage during holographic recording. Additionally, the recording sensitivity of the recording material declines when the monomer concentration is reduced to compensate for shrinkage, as the physical state of the material during recording more closely resembles a glassy polymer in which diffusion rates are substantially reduced. Accordingly, despite Dhar et al. having prepared such materials with increased thicknesses, up to about 1 mm, in order to compensate for the serious decrease in dynamic range exhibited at acceptable levels of transverse shrinkage, the recording sensitivity was still low. When holographic recording media are prepared with increased media thickness, however, then the degree of Bragg detuning, exhibited for any given level of transverse shrinkage, becomes more problematic as a result of the concomitant diminution in the peak width of the Bragg selectivity angular profile. In particular, when the magnitude of Bragg detuning is large relative to the Bragg selectivity peak width, then the observed diffraction efficiency is substantially lower than the value at the Bragg peak, and thus the raw BER of a reconstructed image increases significantly as a result of decreased signal to noise ratio.
In prior art, holographic recording media based upon cationic ring opening polymerization have employed monomers which when homopolymerized produce hard and brittle polymers due to high crosslink density. High crosslink density can act to inhibit attainment of significant extents of polymerization reaction. This is particularly the case for multifunctional moieties where enthalpy values for homopolymerizations can be less than about 50 (kJ/mole epoxide). Previous compositions comprising such multifunctional monomers are described by way of example in U.S. Pat. No. 5,759,721 and in U.S. patent application Ser. No. 08-970,066. Although these monomers exhibit considerably reduced shrinkage upon polymerization, as compared to other monomers such as acrylates, the remaining shrinkage coupled with their relatively unyielding mechanical properties can cause both mechanical and optical difficulties when employed in holographic recording media with thickness of 200 xcexcm or greater. Additionally, inhibition of significant extents of polymerization reaction due to high crosslink density prevents attainment of the full dynamic range of the photopolymerizable medium during holographic recording.
It has now been found that multifunctional epoxide monomers with an epoxy equivalent weight greater than 300 grams per mole of epoxide are suitable for use in holographic recording materials. It has also been found that holographic recording materials comprising certain of these multifunctional epoxide monomers exhibit significantly decreased shrinkage, cracking and brittleness compared with the holographic recording materials of the prior art (Examples 6 and 7). Based on these discoveries, novel multifunctional epoxide monomers, novel holographic recording materials and methods of preparing these novel holographic recording materials are disclosed herein.
One embodiment of the present invention is a holographic recording medium comprising a binder capable of supporting cationic polymerization, an acid generator capable of producing an acid upon exposure to actinic radiation, and a multifunctional epoxide monomer capable of undergoing cationic polymerization and which has an epoxy equivalent weight greater than about 300 (grams/mole epoxide). As discussed below, certain acid generators also require sensitizers. Preferably, the holographic recording medium additionally comprises a difunctional and/or a monofunctional epoxide monomer capable of undergoing cationic polymerization. Optionally, the monofunctional and difunctional monomer have an epoxy equivalent weight less than about 225 (grams/mole epoxide). The holographic medium of the present invention is preferably essentially free from materials capable of undergoing free radical polymerization.
Another embodiment of the present invention is a multifunctional epoxide monomer that undergoes acid initiated cationic polymerization. Each epoxide in the monomer is linked by a group comprising a siloxane to a central silicon atom or alternatively in linked to a central polysiloxane ring, and each monomer and oligomer has an epoxy equivalent weight of greater than about 300 grams/mole epoxide and preferably less than about 1000 grams/mole epoxide. More preferably, the monomer has an epoxy equivalent weight between about 300 and about 700 grams/mole epoxide.
Another embodiment of the present invention is a mixture comprising a binder capable of supporting cationic polymerization and a multifunctional epoxide monomer capable of undergoing cationic polymerization and which has an epoxy equivalent weight greater than about 300 grams/mole epoxide. Preferably, the mixture additionally comprises a difunctional and/or monofunctional epoxide monomer capable of undergoing cationic polymerization and which preferably have an epoxy equivalent weight less than about 225 (grams/mole epoxide). This mixture can be used in the preparation of the holographic recording materials of the present invention by adding thereto the other components of the medium, as described below.
Yet another embodiment of the present invention is a method of preparing the holographic recording medium of the present invention. The method comprises the step of combining the binder, and multifunctional epoxide monomer(s) having an epoxy equivalent weight greater than about 300 grams/mole epoxide and the acid generator. When present, the sensitizer and the difunctional and/or multifunctional epoxide monomer are also combined therewith. Preferably, the binder and multifunctional epoxide monomer having an epoxy equivalent weight greater than about 300 grams/mole epoxide and, when present, the monofunctional and difunctional monomers are combined before adding the other components.
The large epoxy equivalent weight of the multifunctional epoxide monomers used in the holographic recording materials disclosed herein reduces crosslink density, and accordingly a greater extent of polymerization reaction can be achieved during exposure to actinic radiation. Consequently, increased dynamic range can be attained during holographic recording. In addition, holographic recording materials comprising a number of the disclosed multifunctional epoxide monomers exhibit significantly decreased shrinkage, cracking and brittleness compared with the holographic recording materials of the prior art. The monomers and oligomers described here have the advantage of being compatible with the preferred siloxane binders and of exhibiting enthalpy values for homopolymerization that can exceed 75 (kJ/mole epoxide).
Suitable monomers and oligomers for use in the present invention are stable on prolonged storage but capable of undergoing rapid cationic polymerization. They are also miscible with binders used for holographic recording materials. Typically, the monomers and oligomers are siloxanes with epoxy functional groups. The preferred type of epoxy group is a cycloalkene oxide group, especially a cyclohexene oxide group. A xe2x80x9cpolyfunctionalxe2x80x9d monomer or oligomer is a compound having at least three groups of the specified functionality, in the present case at least three epoxy groups. The terms xe2x80x9cpolyfunctionalxe2x80x9d and xe2x80x9cmultifunctionalxe2x80x9d are used interchangeably herein. A xe2x80x9cdifunctionalxe2x80x9d monomer or oligomer is a compound having two groups of the specified functionality; and a xe2x80x9cmonofunctionalxe2x80x9d monomer or oligomer is a compound having one group of the specified functionality.
Polyfunctional monomers of the present invention typically have three or four epoxides (preferably cyclohexene oxide) groupings connected by a linker through an Sixe2x80x94O group, i.e., a xe2x80x9csiloxane groupxe2x80x9d, to a central Si atom. Alternatively, the epoxides are connected by a linker to a central polysiloxane ring. For example, certain polymers and oligomers are represented by Structural Formula (I):
Rxe2x80x3xe2x80x94Sixe2x80x94Rxe2x80x23 xe2x80x83xe2x80x83(I).
Each Rxe2x80x2 in Structural Formula (I) is independently an epoxide substituted aliphatic group connected by an inert linker group comprising a siloxane to the central silicon atom. In one example, Rxe2x80x2 comprises a group represented by Structural Formula (II): 
X1 and X2 are independently an inert linking group. Preferably X2 is xe2x80x94Oxe2x80x94.
Each Ra is independently a substituted or unsubstituted aliphatic group or a substituted or unsubstituted aryl group.
Each Rb is an aliphatic group substituted with an epoxide.
Rxe2x80x3 in structural Formula (I) is Rxe2x80x2 or xe2x80x94H, a substituted aliphatic group, an unsubstituted aliphatic group, a substituted aryl group, an unsubstituted aryl group, a substituted siloxane group, an unsubstituted siloxane group, a substituted polysiloxane group or an unsubstituted polysiloxane group.
In one preferred embodiment, the polyfunctional monomer is represented by Structural Formula (III): 
n is 1, 2, 3 or 4; and R is a substituted or unsubstituted aliphatic group, a substituted or unsubstituted aryl group or is a group represented by Structural Formula (IV) or (V): 
Each Ra, RbX1 and X2 is as defined above in Structural Formula (II); Rc is H, an unsubstituted aliphatic group, a substituted aliphatic group, an unsubstituted aryl group, a substituted aryl group, a substituted siloxane group, an unsubstituted siloxane group, a substituted polysiloxane group or an unsubstituted polysiloxane group.
One example of a polyfunctional monomer represented by Structural Formula (III) is a monomer represented by Structural Formula (VI): 
R in Structural Formula (VI) is a group represented by Structural Formula (VII) or (VIII). 
Each group R1, each group R3 and each group R4 is independently a substituted or unsubstituted C1-12 alkyl, C1-12 cycloalkyl, aryl substituted C1-12 alkyl or aryl group.
Each group R2 is independently a substituted or unsubstituted C1-12 alkylene, C1-12 cycloalkylene, C1-12 arylalkylene, or arylene group, xe2x80x94Y1xe2x80x94[Oxe2x80x94Y1]pxe2x80x94, xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94, xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94Oxe2x80x94Y1Si(Rz)2xe2x80x94Y1xe2x80x94, or xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94.
Each R5 is, independently, an epoxide substituted aliphatic group having 2-10 carbon atoms.
Each group R6 is independently hydrogen, an alkenyl, a substituted or unsubstituted C1-12 alkyl, C1-12 cycloalkyl, aryl substituted C1-12-alkyl or aryl or Rzxe2x80x94(Oxe2x80x94Y1)mxe2x80x94, (Rz)3Sixe2x80x94(Oxe2x80x94Si(Rz)2)qxe2x80x94Y1xe2x80x94 or (Rz)3Sixe2x80x94(Oxe2x80x94Si(Rz)2)qxe2x80x94Oxe2x80x94.
Each Rz is independently a substituted or unsubstituted C1-12 alkyl group, C1-12 cycloalkyl alkyl group, aryl substituted C1-12 alkyl group or aryl group.
Each Y1 is independently a C1-12 alkylene group.
p is an integer from 1 to 5 (preferably 1); m is an integer from 1-10 (preferably 1); and q is an integer from 0 to 4 (preferably 0 or 1).
Preferably, R2 is independently a substituted or unsubstituted C1-12 alkylene, C1-12 cycloalkylene, C1-12 arylalkylene, or arylene group; and each R6 is independently a substituted or unsubstituted C1-12 alkylsilane, C1-12 cycloalkylsilane, C1-12 alkoxysilane, aryl substituted C1-12 alkylsilane, a hydrogen, a vinyl, a substituted or unsubstituted C1-12 alkyl, C1-12 dialkylether (alkyl-O-alkylene-), (C1-12 cycloalkyl)C1-12 alkylether (cycloalkyl-O-alkylene-), C1-12 cycloalkyl, aryl substituted C1-12 alkyl or aryl group.
Specific monomers of this type are those in which R is a group represented by Structural Formula (VII) or (VII) and wherein each group R1, each group R3 and each group R4 is a methyl group; each group R2 is an ethylene, hexylene, or octylene group; each group R5 is a 2-(3,4-epoxycyclohexyl)ethyl grouping, and each group R6 is a hydrogen or ethenyl (xe2x80x94CHxe2x95x90CH2).
Another example of a polyfunctional monomer represented by Structural Formula (III) is a monomer represented by Structural Formula (IX): 
R14 is represented by Structural Formula (X) or (XI): 
Each group R15, each group R17, each group R18, each group R19, each group R20 and each group R22 is independently a substituted or unsubstituted C1-12 alkyl, C1-12 cycloalkyl, aryl substituted C1-12 alkyl or aryl group.
R16 is independently a substituted or unsubstituted C1-12 alkylene, C1-12 cycloalkylene, C1-12 arylalkylene, or arylene group, xe2x80x94Y1xe2x80x94[Oxe2x80x94Y1]pxe2x80x94, xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94, xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94Oxe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94, or xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94.
Each R21 is independently an epoxide substituted aliphatic group having 2-10 carbon atoms.
R23 is independently hydrogen, an alkenyl, a substituted or unsubstituted C1-12 alkyl, C1-12 cycloalkyl, aryl substituted C1-12-alkyl or aryl or Rzxe2x80x94(Oxe2x80x94Y1)mxe2x80x94, (Rz)3Sixe2x80x94xe2x80x94(Oxe2x80x94Si(Rz)2)qxe2x80x94Y1xe2x80x94 or (Rz)3Sixe2x80x94(Oxe2x80x94Si(Rz)2)qxe2x80x94Oxe2x80x94.
Each group X is independently oxygen or R16.
Rz, Y1, m, p and q are as described above.
Preferably, R16 is independently a substituted or unsubstituted C1-12 alkylene, C1-12 cycloalkylene, C1-12 arylalkylene, or arylene group; X is xe2x80x94Oxe2x80x94; and each group R23 is independently a hydrogen, a monovalent substituted or unsubstituted C1-12 alkyl, C1-12 dialkylether (alkyl-O-alkylene-), C1-12 cycloalkyl C1-12 alkylether (cycloalkyl-O-alkylene-), C1-12 cycloalkyl, aryl substituted C1-12 alkyl or aryl group.
Examples of specific monomers represented by Structural Formula (IX) are those in which X is xe2x80x94Oxe2x80x94; R14 is represented by Structural Formulas (X) and (XI); each group R15, R17, R18R19, R20 and R22 is a methyl group; each group R16 is an ethylene, hexylene, or octylene group; each group R21 is a 2-(3,4-epoxycyclohexyl)ethyl grouping; and R23 is a hydrogen, hexyl, or alkylether.
In a second preferred embodiment, the polyfunctional monomer is represented by Structural Formula (XII): 
Each group R7 is independently an unsubstituted aliphatic group, a substituted aliphatic group, an unsubstituted aryl group or a substituted aryl group.
Each group R8 is R9, hydrogen, an alkenyl, a substituted or unsubstituted C1-12 alkyl, C1-12 cycloalkyl, aryl substituted C1-12-alkyl or aryl or Rzxe2x80x94(Oxe2x80x94Y1)mxe2x80x94, (Rz)3Sixe2x80x94xe2x80x94(Oxe2x80x94Si(Rz)2)qxe2x80x94Y1xe2x80x94 or (Rz)3Sixe2x80x94(Oxe2x80x94Si(Rz)2)qxe2x80x94Oxe2x80x94.
Each R9 is independently represented by Structural Formula (II).
Rz, Y1, q and m are as defined above.
In one example of a compound represented by Structural Formula (XII), R7-R13 are defined below.
Each group R7 is independently a substituted or unsubstituted C1-12 alkyl, C1-12 cycloalkyl, aryl substituted C1-12 alkyl or aryl group.
R8 is a defined above. Preferably, R8 is R9 or a substituted or unsubstituted C1-12 alkylsilane, C1-12 cycloalkylsilane, C1-12 alkoxysilane, arylsubstituted C1-12 alkyl silane or a substituted or unsubstituted 1-alkenyl group or a substituted or unsubstituted C1-12 n-alkenyl group where n is greater than or equal to 1.
R9 is represented by Structural Formula (XIII): 
Each group R10 is independently a substituted or unsubstituted C1-12 alkylene, C1-12 cycloalkylene, C1-12 arylalkylene, or arylene group, xe2x80x94Y1xe2x80x94[Oxe2x80x94Y1]pxe2x80x94, xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94, xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94Oxe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94, or xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94Si(Rz)2Y1xe2x80x94. Preferably, each group R10 is independently a substituted or unsubstituted C1-12 alkylene, C1-12 cycloalkylene, C1-12 arylalkylene, or arylene group.
Each Rz, Y1 and p is as defined above.
Each group R11 and R12 is independently a substituted or unsubstituted C1-12 alkyl, C1-12 cycloalkyl, aryl substituted C1-12 alkyl group or aryl group.
Each group R13 is independently an epoxide substituted aliphatic group having from 2-10 carbon atoms.
Specific examples of monomers represented by Structural Formula (XII) include those in which each R7, each R11 and each R12 is a methyl group; R8 is ethenyl (xe2x80x94CHxe2x95x90CH2) or a R9; R9 is represented by Structural Formula (XIII); each group R10 is an xe2x80x94(CH2)2xe2x80x94, xe2x80x94(CH2)6xe2x80x94 or xe2x80x94(CH2)8xe2x80x94; and each group R13 is a 2-(3,4-epoxycyclohexyl)ethyl group.
One example of a difunctional epoxide monomer suitable for use in the disclosed holographic recording materials is a compound represented by Structural Formula (XIV): 
R1 and R5 are as defined above. Compounds of this type are commercially available from Polyset Corporation, Inc., Mechanicsville, N.Y.
Examples of suitable monofunctional epoxide monomers are represented by Structural Formula (XIVa): 
An xe2x80x9cinert linking groupxe2x80x9d is a moiety which: 1) does not react under conditions which induce or initiate cationic polymerization of epoxides; does not interfere with acid initiated cationic polymerization of epoxides; 3) and does not interfere with chemical segregation of binder from polymer formed during cationic polymerization of epoxides. An alkylene group (substituted or unsubstituted), i.e., xe2x80x94(CH2)nxe2x80x94, wherein n is a positive integer, preferably from 1-12, an arylene group (substituted or unsubstituted) and xe2x80x94Oxe2x80x94 are examples of an inert linking group. The term xe2x80x9carylenexe2x80x9d refers to an aryl group which is connected to the remainder of the molecule by two other bonds. By way of example, the structure of a 1,4-phenylene group is shown below: 
Others examples of suitable inert linking groups include (substituted or unsubstituted) an alkylene group (preferably C1-12) in which one or more methylene groups are replaced with a cycloalkyl group (a xe2x80x9ccycloalkylene groupxe2x80x9d), an aryl group (an xe2x80x9carylalkylene groupxe2x80x9d), xe2x80x94Oxe2x80x94 or xe2x80x94Si(Ry)2xe2x80x94, wherein xe2x80x94Ry is as defined above. Specific examples include xe2x80x94Y1xe2x80x94[Oxe2x80x94Y1]pxe2x80x94, xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94, xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94Oxe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94, or xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94Si(Rz)2xe2x80x94Y1xe2x80x94; and Y1, Rz and p are as defined above.
An xe2x80x9calkylsilane groupxe2x80x9d can be represented by (Rz)3Sixe2x80x94Y1xe2x80x94, wherein Rz and Y1 are as defined above, provided that at least one Rz is alkyl; a xe2x80x9ccycloalkylsilane groupxe2x80x9d can be represented by (Rz)3Sixe2x80x94Y1xe2x80x94, wherein Rz and Y1 are as defined above, provided that at least one Rz is cycloalkyl; an xe2x80x9calkoxysilane groupxe2x80x9d can be represented by (Rzxe2x88x92Oxe2x80x94)3Sixe2x80x94Y1xe2x80x94, wherein Rz and Y1 are as defined above, provided that at least one Rz is alkyl; and an xe2x80x9caryl substituted alkylsilane groupxe2x80x9d can be represented by (Rz)3Sixe2x80x94Y1xe2x80x94, wherein Rz and Y1 are as defined above, provided that at least one Rz is an aryl substituted alkyl group.
A xe2x80x9csiloxane groupxe2x80x9d can be represented by (Rz)3SiOxe2x80x94, wherein Rz is as defined above. When at least one alkyl or aryl group represented by Rz is substituted, the siloxane group is said to be substituted. A xe2x80x9cpolysiloxane groupxe2x80x9d can be represented by (Rz)3Sixe2x80x94(Oxe2x80x94Si(Rz)2)qxe2x80x94Oxe2x80x94, wherein Rz is as defined above and q is an integer, preferably from 1 to 4. When at least one alkyl or aryl group represented by Rz is substituted, the polysiloxane group is said to be substituted.
A xe2x80x9cdialkyl etherxe2x80x9d can be represented by Rxe2x80x94Oxe2x80x94Y1xe2x80x94, wherein R is an alkyl group and Y1 is an alkylene group; and a xe2x80x9ccycloalkylalkyl etherxe2x80x9d can be represented by Rxe2x80x94Oxe2x80x94Y1xe2x80x94, wherein R is a cycloalkyl group and Y1 is an alkylene group.
An aliphatic group is a straight chained, branched or cyclic non-aromatic hydrocarbon which is completely saturated or which contains one or more units of unsaturation. Typically, a straight chained or branched aliphatic group has from 1 to about 12 carbon atoms, preferably from 1 to about 8, and a cyclic aliphatic group has from 3 to about 10 carbon atoms, preferably from 3 to about 8. An aliphatic group is preferably a straight chained or branched alkyl group, e.g, methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl or octyl, or a cycloalkyl group with 3 to about 8 carbon atoms. A C1-12 straight chained or branched alkyl group or a C3-8 cyclic alkyl group is also referred to as a xe2x80x9clower alkylxe2x80x9d group. An aliphatic group with one unit of unsaturation is referred to an xe2x80x9calkenylxe2x80x9d group.
Suitable aryl groups groups for momomers of the present invention are those which 1) do not react under conditions which induce or initiate cationic polymerization of epoxides; 2) do not interfere with acid initiated cationic polymerization of epoxides; 3) and do not interfere with chemical segregation of binder from polymer formed during cationic polymerization of epoxides. Examples include, but are not limited to, carbocyclic aromatic groups such as phenyl, naphthyl and biphenyl.
Suitable substituents on an aliphatic group (including an alkylene or alkenyl group) or an aryl group carbocyclic and heteroaryl) are those which 1) do not react under conditions which induce or initiate cationic polymerization of epoxides; 2) do not interfere with acid initiated cationic polymerization of epoxides; 3) and do not interfere with separation of binder from polymer formed during cationic polymerization of epoxides unless the group comprises an epoxide moiety. Examples of suitable substituents include, but are not limited to, halogens, R3Sixe2x80x94, alkyl groups, aryl groups and xe2x80x94OR. Each R is independently a substituted or unsubstituted aliphatic group or a substituted or unsubstituted aryl group, preferably an alkyl group or an aryl group.
The binder used in the process and preparation of the present medium should, of course, be chosen such that it does not inhibit cationic polymerization of the monomers used (e.g., xe2x80x9csupportsxe2x80x9d cationic polymerization), such that it is miscible with the monomers used and the polymerized structure obtained from such monomers, and such that its refractive index is significantly different from that of the polymerized monomer or oligomer (e.g., the refractive index of the binder differs from the refractive index of the polymerized monomer by at least 0.04 and preferably at least 0.09). Binders can be inert to the polymerization processes described herein or optionally can polymerize during one or more of the polymerization events.
Preferred binders for use in the current process are polysiloxanes, due in part to availability of a wide variety of polysiloxanes and the well documented properties of these oligomers and polymers. The physical, optical, and chemical properties of the polysiloxane binder can all be adjusted for optimum performance in the recording medium inclusive of, for example, dynamic range, recording sensitivity, image fidelity, level of light scattering, and data lifetime. The efficiency of the holograms produced by the present process in the present medium is markedly dependent upon the particular binder employed. Although those skilled in the holographic art will have no difficulty in selecting an appropriate binder by routine empirical tests, in general it may be stated that poly(methyl phenyl siloxanes) and oligomers thereof, such as 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane. Examples are sold by Dow Coming Corporation under the tradename DOW Coming 705 (a trimer) and DOW Coming 710 and have been found to give efficient holograms.
The acid generator used in the recording medium of the present invention produces an acid upon exposure to the actinic radiation. The term xe2x80x9cacid generatorxe2x80x9d or PAG is used herein to refer to the component or components of the medium that are responsible for the radiation-induced formation of acid. Thus, the acid generator may comprise only a single compound that produces acid directly. Alternatively, the acid generator may comprise an acid generating component which generates acid and one or more sensitizers which render the acid generating component sensitive to a particular wavelength of actinic radiation, as discussed in more detail below. The acid produced from the acid generator may be either a Bronstead acid or Lewis acid, provided of course that the acid is of a type and strength which will induce cationic polymerization of the monomer. When the acid produces a Bronstead acid, this acid preferably has a pKa less than about 0. Known superacid precursors such as diazonium, sulfonium, phosphonium and iodonium salts may be used in the present medium, but iodonium salts are generally preferred. Diaryliodonium salts have been found to perform well in the present media, with specific preferred diaryliodonium salts being (5-octyloxyphenyl)phenyliodonium hexafluoroantimonate and ditolyliodonium tetrakis(pentafluorophenyl)borate, diphenyliodonium tetrakis(pentafluorophenyl)borate, tolylphenyliodonium tetrakis(pentafluorophenyl)borate and cumyltolyliodonium tetrakis(pentafluorophenyl)borate.
In the absence of any sensitizer, iodonium salts are typically only sensitive to radiation in the far ultra-violet region, below about 300 nm, and the use of far ultra-violet radiation is inconvenient for the production of holograms because for a given level of performance ultra-violet lasers are substantially more expensive than visible lasers. It is well known, however, that by the addition of various sensitizers, iodonium salts can be made sensitive to various wavelengths of actinic radiation to which the salts are not substantially sensitive in the absence of the sensitizer. Such sensitizers include, by way of example, Rubrene. In addition, iodonium salts can be sensitized to visible radiation using certain aromatic hydrocarbons substituted with at least two alkynyl groups or two alkenyl groups, a specific preferred sensitizer of this type being 5,12-bis(phenylethynyl)naphthacene. This sensitizer renders iodonium salts sensitive to the 514.5 nm radiation from an argon ion laser, and to the 532 nm radiation from a frequency-doubled YAG laser, both of which are convenient sources for the production of holograms. Preferably, the sensitizer is photobleachable so that the visible absorption of the holographic medium decreases during exposure.
The proportions of acid generator, sensitizer, binder and monomers in the holographic recording medium of the present invention may vary rather widely, and the optimum proportions for specific components and methods of use can readily be determined empirically by skilled workers. Guidance in selecting suitable proportions is provided in U.S. Pat. No. 5,759,721, the teachings of which are incorporated herein by reference. In general, however, it is preferred that the present medium comprise from about 0.25 to about 5 parts by weight of the monofunctional or difunctional epoxide per part by weight of the polyfunctional epoxide monomer which has an epoxy equivalent weight greater than about 300 (g/mole epoxide). The solution of monomers with binder can comprise a wide range of compositional ratios, preferably ranging from about 90 parts binder and 10 parts monomer or oligomer (w/w) to about 10 parts binder and 90 parts monomer or oligomer (w/w). It is preferred that the medium comprise from about 0.167 to about 5 parts by weight of the binder per total weight of the monomers.
Typically, the medium comprises between about 0.005% and about 0.5% by weight sensitizer, when present, and between about 1.0% and about 10.0% by weight acid generator.
Syntheses of the polyfunctional monomers of the present invention are shown in Schemes 1-5. These syntheses are based on a hydrosilylation in which an olefin is reacted with a silane in the presence of a suitable metal catalyst, such as the Karstedt catalyst (see U.S. Pat. No. 3,715,334), tetrakis(triphenylphosphine)platinum(0), or Wilkinson""s catalyst (RhCl(Ph3P)3). In particular, the syntheses selectively react a single silane group (xe2x80x94Sixe2x80x94H) in a compound comprising two more such groups. Selective hydrosilyation reactions of this type are described, for example, in U.S. Pat. No. 5,484,950, Crivello et al., Polymer Preprints 32:173 (1991), Crivello et al., J. Macromolecular Sciencexe2x80x94Pure and Applied Chemistry A31:1001 (1994) and Crivello and Bi, Polymer Science 32:683 (1994), the entire teachings of which are incorporated herein by reference.
In Scheme 1, one silane group of tetramethyldisiloxane (XV) is reacted with an epoxide substituted aliphatic olefin such as the compound represented by structural formula (XVI). The product, represented by Structural Formula (XVII), can then undergo a second hydrosilylation reaction with tetrakis(vinyldimethylsiloxy)silane, represented by Structural Formula (XVIII), to form the product represented by Structural Formula (XIX). It is noted that this product is encompassed within Structural Formula (VI). Specific conditions for carrying out these reactions are provided in Example 1. 
Polyfunctional epoxide monomers with longer chains can be prepared according to Scheme 2, which utilizes an additional hydrosilylation reaction. Specifically, compound (XVII) is hydrosilylated with a diolefin, preferably with an excess of the diolefin. The product, represented by Structural Formula (XX), then undergoes a second hydrosilylation with tetrakis(dimethylsiloxy)silane, represented by Structural Formula (XXI). The final product, represented by Structural Formula (XXII), is encompassed within Structural Formula (III). R2 in Scheme 2 is xe2x80x94(CH2)n+4xe2x80x94. Specific conditions for these reactions are provided in Example 2. 
Further chain elongations are possible, as shown in Scheme 3. The intermediate represented by Structural Formula (XX) is hydrosilylated with 1,1,3,3-tetramethyldisiloxane (XXIII) to form a product represented by Structural Formula (XXIV), which is then used to hydrosilylate tetrakis(vinyldimethylsiloxy)silane to form a product represented by Structural Formula (XXV). This final product is encompassed within Structural Formula (IX). R16 in Scheme 3 is xe2x80x94(CH2)n+4xe2x80x94; and R16A is xe2x80x94(CH2)2xe2x80x94. 
In Scheme 4, 1-[2-(3{7-oxabicyclo[4.1.0]heptyl})ethyl]-tetramethyldisiloxane (XVII) is reacted with 2,4,6,8-Tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (XXVI) to form the product XXVII, which is encompassed within Structural Formula (XII). Specific conditions for this reaction are provided in Example 3. 
Scheme 5 shows the same synthesis as Scheme 3, except that 1,3-divinyltetramethyldisiloxane is used in place of an aliphatic diene. The product represented by Structural Formula (XXX), is encompassed within Structural Formula (IX). Specific conditions for carrying out this transformation is provided in Examples 4-5. 
The following examples are now given, though by way of illustration only, to show details of particularly preferred reagents, conditions, and techniques used in referred media and processes of the present invention.